Adventures in Thermochemistry. James S. Chickos * Department of Chemistry and Biochemistry University of Missouri-St. Louis Louis MO 63121 E-mail: [email protected] 5. Union Station STL. Previously we concluded the following:.
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Can any more of this be experimentally verified?
Requirements: Vapor pressures of the standards preferably as a function of temperature over a range of temperatures
Using the following series of hydrocarbons as examples:
Retention Times as a Function of Temperature
ta = ti –tCH4
A plot of natural logarithm of the reciprocal adjusted retention times ln(to/ta) for (top to bottom): ,n- octane; , 1-nonene; , n-decane; , naphthalene;
, n-dodecane; , n-tridecane as a function of 1/T; to = 1 min.
of ln(to/ta) versus (1/T)K-1
Compound ln(to/ta)=- slngHm/RT + ln(Ai)
n-octane ln(to/ta)= (-32336/RT) + (11.064) r2=0.9995
1-nonene ln(to/ta)= (-35108/RT) + (11.159) r2=0.9993
n-decane ln(to/ta)= (-38973/RT) + (11.655) r2=0.9994
naphthalene ln(to/ta)= (-41281/RT) + (11.176) r2=0.9997
n-dodecane ln(to/ta)= (-46274/RT) + (12.685) r2=0.9996
n-tridecane ln(to/ta)= (-50036/RT) + (13.232) r2=0.9997
to = 1 min
A plot of experimental vapor pressures ln(p/po) against ln(to/ta) at T = 298.15 K; to = 1 min; po= 101 kPa
slngHm(368 K) ln (A) ln(to/ta) ln(p/po) ln(p/po) ln(p/po)
lita calc lit
octane -32336 11.064 -1.98 -3.99 -3.95
1-nonene -35108 11.159 -3.00 -5.15 -4.96b
decane -38973 11.655 -4.07 -6.32 -6.39
naphthalene -41281 11.176 -5.48 -8.04 -7.98c
dodecane -46274 12.685 -5.98 -8.63 -8.63
tridecane -50036 13.232 -6.95 -9.79 -9.76
ln(p/po) = (1.1820.015) ln(to/ta) -(1.53 0.059); r2 = 0.9987
Vapor pressures for naphthalene are for the liquid
aRuzicka, K.; Majer, V. J. Phys. Chem. Ref. Data 1994, 23, 1-39;
bPhysical Properties of Chemical Compounds II, Dreisbach, R. R. Advances in Chemistry Series 22, ACS, Washington: DC.
cChirico, R. D.; Knipmeyer, S. E.; Nguyen, A. Steele, W. V. J. Chem. Thermodyn. 1993, 25, 1461-4.
Applying this protocol as a function of temperature at T = 15 K intervals and fitting the data for 1-nonene and naphthalene to a third order polynomial results in:
a predicted boiling temperature for nonene of : 421 K (420 K lit)
a predicted boiling temperature for naphthalene of: 507 K (493 K lit)
Vapor pressure of an analyte off a column is inversely proportion to it adjusted retention, 1/ta.
Why is 1/ta proportional to the vapor pressure of the pure material when the enthalpy of transfer is a measure of both the vaporization enthalpy and the interaction on the column?
slngHm(Tm) = lgHm(Tm) + slnHm(Tm)
Daltons Law of Partial Pressures pT = panalyte + pstationary phase = panalyte
Raoult’s Law:: the vapor pressure of component a is equal to the product of vapor pressure of pure a (pao) times its mole fraction, χa
pa(obs) = pao·χa
Since the stationary phase is a polymer, χa≈ 1
The effects of slnHm(Tm) are small and compensated by the standards.
Returning to the n-alkanes
po = 101.325 kPa
ln(to/ta) = -slnHm(Tm)/R*1/T + intercept
Vapor pressures of n-alkanes (C14 to C20) at T = 298.15 K:
po = 101.325 kPa
ln(p/po) = (1.27 0.01) ln(to/ta) - (1.693 0.048); r 2 = 0.9997
Vapor pressure -temperature dependence for hexadecane;
line: vapor pressure calculated from the Cox equations for C14,
circles; vapor pressures calculated by correlation treating hexadecane as an
unknown and correlating ln(to/ta) with ln(p/po) for C14, C15, C17-C20 as a function
of temperature from T = (298.15 to 500) K.
Normal boiling temperature: 560.2 (expt); 559.9 (calcd)
By such a process of extrapolation, vapor pressure equations were obtained for C21 through to C38 using commercially available samples from T = (298.15 to 540) K at 30 K intervals and the resulting vapor pressures were fit to the following third order equation which has been found to extrapolate well with temperature:
ln(p/po) = A (T/K)-3 + B(T/K)-2 + C(T/K)-1 + D;
Using this equation the boiling temperatures of C21 to C38 could be predicted
aLiterature value. b This work. c Mazee, W. M., “Some properties of hydrocarbons having more than twenty carbon atoms,” Recueil trav. chim 1948, 67, 197-213. Francis, F.; Wood, N. E., The boiling points of some higher aliphatic n-hydrocarbons, J. Chem. Soc. 1926, 129, 1420.
Using vapor pressures calculated from C24 through to C38, values for C40 through to C76 were evaluated.
PERT2 is a FORTRAN program written by D. L. Morgan in 1996 which includes parameters for n-alkanes from C1 to C100 and heat of vaporization and vapor pressure correlations. The parameters for C51 to C100 are unpublished based on the critical property (Tc, Pc) correlations of Twu and the Kudchadker & Zwolinski extrapolation of n-alkane NBPs presented in Zwolinski & Wilhoit (1971).
a Morgan, D. L.; Kobayashi, R. Extension of Pitzer CSP models for vapor pressures and heats of vaporization to long chain hydrocarbons.Fluid Phase Equilib. 1994, 94, 51–87.
b Kudchadker, A. P.; Zwolinski, B. J. Vapor Pressures and Boiling Points of Normal Alkanes, C21 to C100. J. Chem. Eng. Data 1966, 11, 253.
ln(p/po) = A(T/K)-3 + B(T/K)-2 +C(T/K) + D
A plot of the normal boiling temperatures of the n-alkanes as a function of the number of methylene groups resulted in the following:
N = the number of carbon atoms. The solid symbols represent the experimental and the others the calculated boiling temperatures of C3 to C92. The dotted line was calculated for the n-alkanes using a limiting boiling temperature of TB(∞) = 1076 K. The solid line was obtained by using a by fitting the experimental data to the hyperbolic function previously described and a value of TB(∞) = (1217 ± 246) K
Based on the data available, it appears that boiling temperature appear consistent with the prediction that boiling temperatures would approach a limiting value. The agreement with average value of 1217 obtained previously is probably fortuitous